The present invention relates to vehicles having a continuously variable transmission (CVT), known also as an infinitely variable transmission, and more specifically to agricultural and industrial vehicles having a CVT, and even more specifically to tractors having a CVT.
Agricultural and industrial vehicles can take many forms, such as backhoe loaders, wheel loaders, forestry fellers, skidders, harvesters, sprayers, application equipment, utility vehicles, agricultural tractors, and compact tractors. Often, these agricultural and industrial vehicles are required to perform tasks at low speeds with a high amount of tractive output, typically off-road, under which circumstances the range of speeds involved are all under a comparatively low limit, and under which circumstances a significant variation of the control pedal or lever corresponds to a small variation of the vehicle speed. These same agricultural and industrial vehicles may then be required to travel on-road at relatively high speeds, wherein the range of speeds involved are under a significantly higher limit, and under which circumstances a small variation of the control pedal or lever corresponds to a significant variation of the vehicle speed.
In a traditional on-road vehicle, such as a car or truck, the vehicle speed is controlled by varying the application of the throttle and by varying the transmission ratio. In an on-road vehicle having a manual transmission, the vehicle speed is controlled within the range of speeds for a selected gear by such application of the throttle. In an on-road vehicle having an automatic transmission, the vehicle speed is controlled over its entire range of speeds by such application of the throttle, with the transmission selecting and engaging the appropriate gear according to the velocity and the commanded acceleration. In a traditional agricultural or industrial vehicle, however, for example a tractor, the operator selected a transmission gear and set the throttle for a given speed, which did not largely vary once the clutch was engaged. A greater amount of tractive effort could then be achieved by application of a lower gear and a higher throttle setting, with the operator making such selections. In other words, in on-road vehicles, the engine is typically controlled in torque, whereas in agricultural and industrial off-road vehicles, the engine is typically controlled in speed.
Infinitely variable transmissions allow the selection of any transmission ratio within a given range. Infinitely variable transmissions operate on various principles, including variable diameter pulleys, toroidal CVTs, and hydrostatic CVTs. In agricultural and industrial vehicles, hydrostatic CVTs dominate due to their ability to transmit a great amount of torque. In a traditional agricultural or industrial vehicle having a manually controlled CVT, the operation of the vehicle was not entirely dissimilar from operating a manual transmission agricultural or industrial vehicle with a clutch. That is to say, the throttle was typically set to a given engine speed, commonly referred to as engine revolutions per minute (ERPM), and the vehicle speed was varied using a lever or a drive pedal that operated the CVT.
More advanced agricultural or industrial vehicles have an automatically controlled CVT, sometimes paired with a geared transmission, and an automatically controlled ERPM. Such vehicles having an automatically controlled CVT and an automatically controlled ERPM are known for example from EP 1 156 942 B1. In this configuration, the operator changes the desired vehicle speed by using the lever or the drive pedal. A CVT controller then accomplishes the desired vehicle speed by modifying both the ERPM and the CVT output ratio. The CVT controller does this by sending an ERPM request signal to an Engine Control Module (ECM) by way of a Controller Area Network (CAN) bus. In an agricultural or industrial vehicle having a CVT paired with a geared transmission, the CVT controller may further manage the geared transmission ratio using electrovalves controlling clutches, synchronizers, and the pump volume of the hydrostatic unit.
Even with advanced agricultural or industrial vehicles having a CVT with an automatically controlled transmission ratio and automatically controlled ERPM (AutoMode), certain difficulties can arise with the responsiveness of the vehicle to the operator's intentions. Particularly, not only do the range of speeds and acceptability of speed variation differ between use off-road and use on-road, but the expected response in terms of acceleration and deceleration to lever or drive pedal position changes differs under these varied circumstances.
What is needed in the art is a system that provides for improved drivability and better correlation between an operator's inputs and the operator's expected responses from the agricultural or industrial vehicle in terms of speed and acceleration or deceleration.